The invention is concerned with a procedure for the correction of error in regard to speed related measurement values, in particular, the thickness of a fiber band in a textile machine, more particularly, in a stretch works such as a drawframe.
DE 44 41 067 concerns a controlled stretch works for fiber bands with a feed element for a plurality of incoming fiber bands. In this case, there is available a delay mechanism, a drive system, and a control or regulating means for the drive system. The control, or regulation, reacts to a measurement signal that is sent by the feed element to change the delay of the fiber band by the drive system, so that weight variances in the feed fiber band can be corrected. The regulated stretch works should make possible an improved tendency toward uniformity of the fiber bands, especially by a change in the speed of the feed delivery system by use of braking and acceleration.
The measurement signal of the feed element should be made to conform to and depend on the operating conditions to compensate for changes to the measurements caused by the operating conditions. The state of the technology proceeds from the standpoint of correcting the measurement signal from the feed element on the basis of the output speed (see in said Patent Column 2, lines 52 to 54). This output speed is, however, determined at the exit rolls at the end of the stretchworks arrangement (relative to the calender rolls behind the band receiving hopper.) This approach to speed measurement arises from the conventional concept of the present practice. On the feeler rolls, the speed of the fiber band cannot be determined, because first, delay adjustments are made as part of the control, and second, the feeler roll pair are mechanically interlocked with the delay roll pair. This left only the conclusion to be made that the speed of the fiber band had to be measured at the exit end of the stretch works.
So, from the values of the output speed and the values of the delay, the intake feed speed of the fiber band was then determined by calculation.
This former method of thinking was based on the idea that at the feed entry, speed changes caused by regulation were not available for a measuring instrument. That is, the entry speed as well as the exit speed were assumed to remain equal.
This assumption did not consider the actual available speed changes and the errors which arose therefrom. The use of the delivery speed in the correction which is proposed by the previous state of the technology cannot find support in the actual factual situation. Doing so, leads to the point that an error in the incoming band can be as much as .+-.25% of the thickness. For instance, at an error of 25%, a rotational speed change of 33% relative to the nominal delay can occur. The use of the exit speed according to the state of the technology again is not supported by the actual body of facts. That described correction process is thus not adaptable to a modern stretch works.
The measurement element is a measuring instrument which actually touches the fiber material. In the spinning world, such measurement elements are known as "feeler rolls", or a feeler probe. A characteristic of the measuring instrument is that a means of sensing, for instance a pivotable roll in a touch roll pair, or a movable probe of a conical sensing probe, touches the moving fiber material. The sensing means is pressed with a specified pressure against the fiber material. The back-thrust from the material is transduced into an electrical measurement signal, the measurement value of which corresponds to the measured thickness of the fiber material.
This kind of measuring element is installed on spinning machines to determine the thickness of the fiber material. Such an element is, for instance, customary for the regulation of the draw works for carding, drawing, or ring spinning machines as well as in the regulation of the entry of fiber material into the spinning box of a rotor spinning machine. The measurement value output by said element is sent to a regulating means which controls the operation center of a spinning machine. The development of a spinning machine with higher productivity is accompanied by an increase in the speed of the fiber material. In place of spinning machines, this advance in regard to a stretch machine will serve to explain the development.
From an original delivery speed of 850 m/min, a speed of 1000 m/min has been achieved, in the case of delivery from modern stretch works. Accordingly, the speed at the feed end is substantially increased. Since regulated stretchworks have a measuring instrument for the control of the delay, in every case, an increasing speed of the fiber material at the measuring instrument is relevant.
It is generally recognized that a disturbing error in the measurement signal is produced when the speed of fiber material is in a startup phase, or the fiber material finds itself at a standstill. This is also true in regard to speed changes at the measuring instrument as a result of the regulation of the delay. It becomes evident, that upon acceleration or braking of the movement of the fiber material, the error of the measured signal will be just that much greater, in direct proportion to the speed difference of the fiber material to be measured.
The disadvantage of this is, that in the presence of speed differences with the moving fiber material, a misleading measurement signal is produced. Upon further conditioning in the control apparatus of a spinning machine, this signal is noticeably disturbing, lending to faulty regulation.
In the case of regulated drawframe, this situation brings about faultily tensioned fiber material during startup, as well as at standstill or during speed changes at the time of can exchange. This yields clear deviations in the band number as opposed to the band number derived from the operational speed.
This quantitative embracing of the speed related error in the measurement signal first became clear upon the installation of digital technology. As a result of a more exact computational ability for the evaluated measurement values, the quantitative effect of the error became noticeable. Previously, the error was always estimated as negligible. From this background, questions arose in regard to the reason for the error.
The fiber material, as occurs with natural fibers, has a fuzziness, a roughness, or hairiness. On this account, there are air inclusions between the fibers. Upon increasing speed of the fiber material, the interfering influence of this factor becomes evident at an increasing rate, in spite of uniform pressure of the feeler means on the fiber material.